Contact isolated potentiometer



Oct. 30, 1962 L. E. HOLLANDER, JR

CONTACT ISOLATED POTENTIOMETER Filed Aug. 23, 1961 "oil 6 IO n n l6 8 1 F|G.1 20 E 1 I f H 34 It C T 1 f il E 40 E 1 22 &

5 Sheets-Sheet 1 FIG. 3

III'IVIIIIII INVENTOR. LEWIS E. HOLLANDER, JR.

f Agent Oct. 30, 1962 L. E. HOLLANDER, JR 3,061,807

CONTACT ISOLATED POTENTIOMETER 5 Sheets-Sheet 2 Filed Aug. 23, 1961 mmvrox LEWIS E. HOLLANDER JR! BY Oct. 30, 1962 L. E. HOLLANDER, JR 3,061,807

CONTACT ISOLATED POTENTIOMETER Filed Aug. 23, 1961 5 Sheets-Sheet 3 RESISTIVITY A-AXIS RESISTIVITY C-AXIS RESISTIVITY C-AXIS m OHM-CM ll 9 l.- b g: o

D. o E o 2 Z ANISOTROPY RATIO INVENTOR.

LEWIS E.HOLLAN DER JR.

BY Z

' Agent FIG-9 United States Patent Ofitice 3,061,807 Patented Oct. 30, 1962 3,061,867 CONTACT ISOLATED POTENTIOMETER I Lewis E. Hollander, In, Los Altos Hills, Calif, assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Aug. 23, 1961, Ser. No. 133,501 13 Claims. ('Cl. 338176) This invention relates to potentiometers and more particularly to potentiometers in which the direct contact of the resistive material by a moving part is eliminated.

In all existing potentiometers or variable resistors of which I have knowledge direct contact with the resistive material by a moving part has been necessary. Such contact is disadvantageous because the resistive material is worn away by such contact and/or substantial electrical noise is generated. Additionally, in such prior art devices it has been impossible to completely seal the re sistive material from atmospheric conditions with the result that the resistive material becomes rapidly contaminated when the potentiometer is used under adverse atmospheric conditions. In order to construct a wirewound potentiometer having a high resistance it is neces sary to use Wire having a very thin diameter, which wire is very fragile and subject to rapid wear. Potentiometers having a thin layer of resistive material partially obviate the last-mentioned disadvantage, but the film of resistive material is subjected to wear as the moving contact rubs on the thin film such that the resistance changes as wear proceeds.

Therefore it is an object of my invention to provide a potentiometer in which no moving contact with the resistive film is required.

Another object is to provide a potentiometer having a higher electrical resistance than was feasible heretofore.

Still another object is to provide a potentiometer in which the resistive film can be completely sealed from exposure to the atmosphere. 7

Yet another object is to provide a potentiometer in which the surface on which the resistive film is placed can be optically polished thereby providing improved thin film properties of the resistive element.

These and other objects will be more apparent after referring to the following specification and attached drawings in which:

FIGURE 1 is a plan view of an embodiment of my invention taken along line I-I of FIGURE 2;

FIGURE 2 is a view taken along line IIII of FIG- URE 1;

FIGURE 3 is a schematic diagram of the equivalent electrical circuit of my device;

FIGURE 4 is a perspective view of a modification of the embodiment of FIGURES 1 and 2;

FIGURE 5 is a perspective view of the preferred embodiment of my invention;

FIGURE 6 is a view taken on line VI-VI of FIG- URE 5;

FIGURE 7 is a plan view of another embodiment of my invention;

FIGURE 8 is a view taken on line VIII-VIII of FIG- URE 7; and

FIGURE 9 is a graph of anisotropy ratio versus resistivity in ohm-centimeters of single-crystal rutile.

As more fully explained in my co-pending application for United States Letters Patent, Serial Number 39,584, filed June 29, 1960, I have discovered that single-crystal rutile when appropriately reduced, possesses substantial anisotropic conduction properties. My present invention utilizes this property in providing an improved potentiometer.

Referring more particularly to the drawings, in FIG- URES 1 and 2, reference numeral 2 indicates a body of reduced single-crystal rutile suitably mounted on an insulated base 4 having vertical members 6 and 8 attached thereto. Rutile body 2 has two opposed substantially parallel surfaces 10 and 12 which are substantially parallel to the a crystallographic axis and are substantially normal to the c crystallographic axis of the body 2. Surface 12 is preferably polished to optical smoothness and has coated thereon a thin film 14 of any well-known resistive material, for example, a substance produced under the trade name Even-Ohm. The dimensions of film 14 may be readily chosen by those skilled in the art to obtain the desired resistance value. Attached to opposite ends of resistive film 14 are electrodes 16 and 18 to which wires 20 and 22, respectively, are soldered. Electrodes 16 and 18 may be of any suitable material, for example, gold or silver. A layer 24 of any suitable insulative sealing or potting material such as silicon monoxide is placed over film 14 and contacts 16 and 18 for protection from atmospheric conditions. A guide bar 26 is supported between vertical members 6 and 8 and is disposed parallel with surface III. A threaded rod 28 is suitably journaled in vertical members 6 and 8 substantially parallel to guide 25 and has an enlarged end portion 30 exterior of vertical member 8. A movable contact 32 has a hole 34 therethrough shaped to receive the guide 26 and a threaded hole 36 to receive rod 28. Lead wire 38 is connected to one end of the rod 26.

The operation of this embodiment of my invention is as follows: with rutile body 2 reduced so as to have the resistivity in the direction of the c axis in the range of about from 10 to 10 ohm-centimeters, the resistivity in the direction of the c axis will be much less than the resistivity in the direction of the a axis As may be seen from FIGURE 9, if rutile body 2 is reduced so as to have a resistivity in the direction of the c axis equal to about 10 ohm-centimeters, the resistivity in the direction of the a axis will be approximately 10 times the resistivity in the direction of the c. There are many processes for reducing rutile, one of which is fully described in commonly assigned co-pending application for United States Letters Patent, Serial Number 56,140, filed September 15, 1960. The ratio of resistance in the a direction to resistance in the c direction can be further increased by proper design of the geometry of the rutile body 2. By maintaining the dimension in the direction parallel with the c axis small (i.e., the distance between surfaces 10 and 12 of the body 2) the resistance in series with the contact 2 may be minimized and the resistance in parallel with the resistive film 14 may be maximized. For example, if the thickness of rutile body 2 is chosen to be approximately 1 millimeter, the resistance in the c direction will be approximately ohms. If the length of rutile body 2 is chosen to be approximately 1 centimeter, the resistance in the direction of the a axis will be approximately 10 ohms. Thus, if resistive film 14 is chosen to have a resistance between the terminals 16 and 18 of 10 ohms, movement of contact 32 across surface 10 of rutile body 2 by rotation of enlarged end portion 30 will cause the resistance between wires 20 and 38 to vary between approximately 100 ohms and 100,000 ohms. The foregoing may be understood more clearly by a reference to FIGURE 3 in which 2' corresponds to the resistance along the c axis of rutile body 2, 14' represents the resistance of film 14, and 4t) represents the resistance of rutile body 2 in the direction of the a axis. Terminal 16' of FIGURE 3 corresponds with electrode 16, terminal 18 corresponds with the electrode 18, and terminal 32' corresponds to movable contact 32 or wire 38. It may be seen, therefore, that when a potentiometer is constructed having the parameters set forth next above, the minimum value of resistance obtainab le is approximately 100 ohms, while the maximum value is approximately 100,000 ohms less about 1% due to the shunting eifect of resistance 40 due to resistivity of rutile body 2 in the direction of the :1 axis. It will be obvious to those skilled in the art that the dimensions of rutile body 2 and the amount by which the rutile is reduced may both be varied to obtain potentiometers with different resistance ranges. To construct a 1,000 ohm potentiometer I prefer to reduce the single-crystal rutile so that it possesses a resistivity of approximately 100 ohmcentimeters. From FIGURE 9 it Will be seen that a rutile body so reduced will have an anisotropy ratio of approximately 500 to 1. If the rutile body 2 is then cut such that the dimension in the c direction is 1 millimeter and the dimension in the a direction is 1 centimeter and a resistive film 114 having a resistance of 1,000 ohms is placed on the surface 112 a potentiometer results which has approximately 10 ohms in series with the movable contact and in excess of 100,000 ohms in parallel with the 1,000 ohms resistive film.

In addition to the anisotropic conduction characteristic of single-crystal rutile, the material has the added advantage that it may be polished to an almost optically perfect smoothness, with the result that the resistive film 14- may be placed thereon with high accuracy so as to assure reproducibility.

FIGURE 4 indicates a modification of the device described above and has a plurality of electrical contacts 42 on the surface 10. The contacts 42 may be of any suitable conductive material such as titanium metal or silver. The presence of contacts 42 improves the noise characteristics of the device, but of course introduces the disadvantage that the resistance setting of the potentiometer is not continuously variable.

I prefer to construct my improved potentiometer by providing a split annular body of single-crystal rutile 44 as shown in FIGURES and 6. Rutile body 44- is disposed such that the c crystallographic axis is substantially parallel to the annular axis, with surfaces 46 and 48 on rutile body 44 being substantially normal to the c axis. Rutile body 44 may be supported in any suitable manner such as in a ring of insulative material 50. Surface 43 is preferably polished to optical smoothness and has an annular resistive coating 52 and a coating of sealing material 54 placed thereon in a manner described above. Adjacent each side of the split in annular body 44 is an electrode 56, each electrode 56 having a wire .28 soldered thereto. Attached to the upper surface of sup port member 50 is a bracket 60 for supporting one end of a shaft 62, and on the lower face of support member 50 is a bracket 64 for supporting the opposite end of shaft 62. Attached to shaft 62 for rotation therewith is a movable contact or wiper 66 for sliding engagement with surface 46 of rutile body 44. Electrical connection to movable contact 66 may be made in any suitable fashion such as by connecting a wire 68 to bracket 60.

The theory of operation of this embodiment of my invention is identical with that described hereinabove, but in the case of the present embodiment the resistive film is disposed in a circular manner thereby conserving space. The preferred embodiment has the advantages that the movable contact 66 does not rub against resistive film 52 to Wear away the film and the resistive film 52 is protected by layer 54 from atmospheric contamination. The describe/.1 means for supporting the split annular body of rutile 44 is merely exemplary and may be modified in many way well known to those skilled in the art.

The embodiment of FIGURE 5 may be modified as shown in FIGURES 7 and 8 by providing a plurality of spaced-apart contacts 70 on the surface 46 of rutile body 44. The contacts '70 may be any suitable conductive material such as titanium metal or silver and may be spaced from one another at any suitable distance. The width of the contacting surface of movable contact 66 may be such that it contacts only one fixed contact 70 at a given time or such that it makes contact with the next adjacent contact '70 before leaving the preceding contact 70'. This embodiment would have the advantage of low contact resistance but the disadvantage of discontinuous variation of resistance.

In constructing my improved potentiometer, I prefer to cut a body of the proper shape and alignment with respect to the crystallographic axes from a boule of singlecrystal rutile. One of the surfaces of the rutile body normal to the c crystallographic axis is then polished to optical smoothness using conventional techniques. Thereafter I place the rutile body in a vacuum chamber and process the rutile until it is reduced to the required anisotropy in accordance with the curve of FIGURE 9. When the reduction process is complete and while the body is still in the vacuum chamber, I place the resistive film on the polished surface, place the gold electrodes on each end of the film, solder the Wires to the electrodes and evaporate a coating of silicon monoxide or the like upon the resistive coating. I then remove the prepared body from the vacuum chamber, clean the opposite surface of the body and assemble it as a potentiometer. In this manner, the resistive coating is never exposed to contaminants which may exist in the atmosphere.

While several embodiments of my invention have been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim as my invention.

1. A potentiometer comprising a body of reduced single-crystal rutile, said body having a first surface in a plane normal to the c crystallographic axis and a second surface substantially parallel to said first surface, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a movable contact for engaging said second surface, and means for moving said contact from a point on said second face substantially opposite said first electrode to a point on said second surface substantially opposite said second electrode.

2. A potentiometer comprising a body of single-crystal rutile reduced so as to possess a resistivity in a direction along the c crystallographic axis of said body in the range of about 10 ohm-centimeters to 10" ohm-centimeters, said body having a first surface in a plane normal to the c crystallographic axis and a second surface substantially parallel to said first surface, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a movable contact for engaging said second surface, and means for moving said contact from a point on said second face substantially opposite said first electrode to a point on said second surface substantially opposite said second electrode.

3. A potentiometer comprising a body of single-crystal rutile reduced so as to possess maximum anisotropy of conduction between two mutually perpendicular axes of said body, said body having a first surface in a plane normal to the c crystallographic axis and a second surface substantially parallel to said first surface, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a movable contact for engaging said second surface, and means for moving said contact from a point on said second face substantially opposite said first electrode to a point on said second surface substantially opposite said second electrode.

4. A potentiometer comprising an elongated body of single-crystal rutile reduced so as to possess a resistivity in a direction along the c crystallographic axis of said body in the range of about 10 ohm-centimeters to 10' ohm-centimeters, said body having a first surface parallel with the axis of elongation and substantially normal to the c crystallographic axis, a second surface substantially parallel to said first face, a coating of resistive material on said first surface, a first conductive electrode connected to one end of said coating, a second conductive electrode connected to the end of said coating opposite said first electrode, a movable electrode for slidably contacting said second surface and means for supporting said movable electrode.

5. A potentiometer comprising an elongated body of single-crystal rutile reduced so as to possess maximum anisotropy of conduction between two mutually perpendicular axes of said body, said body having a first polished surface parallel with the axis of elongation and substantially normal to the c crystallographic axis, a second surface substantially parallel to said first face, a coating of resistive material on said first surface, a first conductive electrode connected to one end of said coating, 21 second conductive electrode connected to the end of said coating opposite said first electrode, a movable electrode for slidably contacting said second surface and means for supporting said movable electrode.

6. A potentiometer comprising a body of single-crystal rutile reduced so as to possess a resistivity in a direction along the c crystallographic axis of said body in the range of about ohm-centimeters to 10 ohm-centimeters, said body having a first surface in a plane normal to the c crystallographic axis and a second surface substantially parallel to said first surface, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a plurality of spaced apart fixed contacts on said second surfaces disposed opposite said resistive coating and between points opposite said first and second electrodes, and means for selectively contacting at least one of said contacts.

7. A potentiometer comprising a body of single-crystal rutile reduced so as to possess maximum anisotropy, of conduction between two mutually perpendicular axes of said body, said body having a polished first surface in a plane normal to the c crystallographic axis and a second surface substantially parallel to said first surface, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a plurality of spaced apart fixed contacts on said second surfaces disposed opposite said resistive coating and between points opposite said first and second electrodes, and means for selectively contacting at least one of said contacts.

8. A potentiometer comprising a split annular body of single-crystal rutile reduced so as to possess a resistivity in a direction along the c crystallographic axis of said body in the range of about 10 ohm-centimeters to 10 ohm-centimeters, said body being disposed so that the annular axis is substantially parallel to the c crystallographic axis of said body and having a first surface substantially normal to said axis and a second surface spaced from said first surface substantially parallel thereto, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a movable contact for engaging said second surface, and means for moving said contact from a point on said second face substantially opposite said first electrode to a point on said second surface substantially opposite said second electrode.

9. The invention of claim 8 in which said contact moving means comprises a rotatable shaft disposed substantially coaxially with the annular axis of said body.

10. A potentiometer comprising a split annular body of single-crystal rutile reduced so as to possess maximum anisotropy of conduction between two mutually perpendicular axes of said body, said body being disposed so that the annular axis is substantially parallel to the c crystallographic axis of said body and having a first polished surface substantially normal to said axis and a second surface spaced from said first surface substantially parallel thereto, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a movable contact for engaging said second surface, and means for moving said contact from a point on said second face substantially opposite said first electrode to a point on said second surface substantially opposite said second electrode.

11. A potentiometer comprising a split annular body of single-crystal rutile reduced so as to possess a resistivity in a direction along the c crystallographic axis of said body in the range of about 10 ohm-centimeters to 10" ohm-centimeters, said body being disposed so that the annular axis is substantially parallel to the 0 crystallographic axis of said body and having a first surface substantially normal to said axis and a second surface spaced from said first surface substantially parallel thereto, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a plurality of spaced apart fixed contacts on said second surfaces disposed opposite said resistive coating and between points opposite said first and second electrodes, and means for selectively contacting at least one of said contacts.

12. The invention of claim 11 in which said selective contacting means comprises a rotatable shaft disposed substantially coaxially with the annular axis of said body, and a wiper connected for rotation with said shaft for contacting at least one of said fixed contacts.

13. A potentiometer comprising a split annular body of single-crystal rutile reduced so as to possess maximum anisotropy of conduction between two mutually perpendicular axes of said body said body being disposed so that the annular axis is substantially parallel to the c crystallographic axis of said body and having a first polished surface substantially normal to said axis and a second surface spaced from said first surface substantially parallel thereto, a coating of resistive material on said first surface, a first conductive electrode connected to said coating, a second conductive electrode connected to said coating spaced from said first electrode, a plurality of spaced apart fixed contacts on said second surfaces disposed opposite said resistive coating and between points opposite said first and second electrodes, and means for selectively contacting at least one of said contacts.

References Cited in the file of this patent UNITED STATES PATENTS 2,940,941 Dalton June 14, 1960 

